linear over the range 40 to 400 pg Ce-ie., 1.6 to 16 pg ml-1 of cerium in the initial aqueous solution. The absorbance values which correspond to these concentrations in the aqueous solution are 0.07 and 0.73. A sensitivity for cerium by this indirect method of 0.093 pg ml-1 (for 1% absorption) was obtained. The procedure blank was reproducible and gave rise to an absorbance of 0.04 at 313.2 nm us. an isobutyl acetate solvent blank. The virtually quantitative extraction of phosphate into the isobutyl acetate medium was demonstrated by taking several different known weights of cerium through the procedure. The MPA in the final isobutyl acetate phases was decomposed and back-extracted by shaking with 10 ml of 4M ammonia solution. The solutions were then diluted to 25 ml with distilled water. The concentration of molybdenum in the ammoniacal phase was determined by AAS using an AAS calibration graph prepared from aqueous molybdate solutions which were also prepared to contain 4M ammonia. The results of these experiments gave a ratio of cerium to final measured molybdenum of 1:5:9 f 0.2. The Mo:Ce ratio of 6:1, the well-known 12:l Mo:P ratio in MPA (14) and the 2 : l ratio in MCPA (11) confirms that MCPA is quantitatively formed in the initial aqueous phase, that MCPA is not extracted by the nbutano1:chloroform solvent used to remove excess MPA, and the complete extraction as MPA of the phosphorus associated with the cerium after decomposition of MCPA into the single isobutyl acetate aliquot used for the final extraction. An estimate of the precision of the recommmended method was obtained from the results obtained for ten samples each containing 200 pg ml-1 of cerium. These samples gave a mean absorbance value of 0.388. The stan-
dard deviation was 0.018 which corresponds to a relative standard deviation of 4.7%. A similar interference pattern to that in the indirect ultraviolet spectrophotometric method %’as found for the indirect AAS method because the selectivity results from the high selectivity of the solvent extraction step with isobutyl acetate. CONCLUSION Two solution spectrophotometric methods are reported for the determination of cerium via the formation of MCPA. The AAS method reported is approximately 900 times more sensitive than the direct method for cerium in a nitrous oxide-acetylene flame (1%absorption is given by ca. 83 pg ml-1 of cerium by the direct method, whereas by this method 1%absorption results with an initial aqueous cerium solution of 0.093 p g m1k1). The enhancement is achieved not only because the AAS sensitivity is greater for molybdenum than cerium but also due to the fact that six molybdenum atoms are associated with each cerium atom; solvent extraction of the measured molybdenum also increases the nebulization efficiency as well as the molybdenum concentration in the final solution. The method is less sensitive than the solution spectrophotometric methods based on MCPA but is more rapid than the indirect ultraviolet molecular absorptiometric method. Received for review October 11, 1972. Accepted January 15, 1973. We are grateful to Alcan Research and Development Ltd. for financial support of this work and t o the Science Research Council for the grant of a CAPS studentship to one of us (H.N.J.).
Direct Determination of Lead Airborne Particulates by Nonflame Atomic Absorption J. P. Matoukek and K. G. Brodie
Varian Techtron Pty. Ltd., North Springvale, Victoria, 3 171, Australia
The direct determination of airborne lead particulates was performed by atomic absorption using nonflame atomization. Particulate matter was collected on a disk of Millipore filter (pore size 0.22 p m ) which had been previously inserted in a modified graphite sampling cup. An air sample volume of 200 ml was sufficient. Prior to the analysis an excess of phosphoric acid was added to the sample in order to produce a single lead absorption peak. The absolute sensitivity achieved (for 1% absorption) was 1.7 X l o - ’ ’ gram of lead which represents 0.1 p g Pb/m3 for a 200-ml air sample. It was shown that aqueous lead solutions added to the standard graphite cup could be used for standardization. The re1 std dev calculated for aqueous solutions (equivalent to a level of 1 p g Pb/m3 in air) was 4.2%. Results determined by this method using aqueous standards correlated well with those by a conventional method. 1606
Atmospheric contamination by lead from automobile exhausts presents a potential hazard to human health. According to Schroeder and Nason ( I ) , the use of lead additives to gasoline has resulted in human body burdens of 120-480 mg of lead. A number of methods for analysis of atmospheric lead have been suggested-a summary of such methods has been given by Loftin et al. ( 2 ) . Such conventional methods require sampling of large volumes of air to obtain an adequate analytical signal. The volume requirement can be drastically reduced by using nonflame atomic absorption. The inherent sensitivity of nonflame methods makes them eminently suitable for such trace metal analysis. Collection of lead particulates on a filter from air vol(1) H. A. Schroederand A. P. Nason, Clin. Chem.. 17,461 (1971). (2) H. P. Loftin. C. M. Christian, and J. W. Robinson, Spectrosc. Lett.. 3, 161 (1970).
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 9, A U G U S T 1973
umes down to 10 1. followed by chemical digestion and nonflame atomic absorption has been reported ( 3 ) . Further reduction in the sample volume and also elimination of chemical treatment was subsequently achieved with the carbon rod atomizer ( 4 ) and a graphite tube furnace ( 5 ) . The continuous determination of lead in air has been achieved by combining nonflame atomization witb a long path absorption tube (2). This paper describes a rapid and sensitive method for particulate lead analysis from sub-liter sample volumes.
,--Tube
Support Rod? CUP
//-
EXPERIMENTAL Apparatus. A Varian Techtron AA-5 spectrophotometer and G-2000 Varian 10-mV chart recorder were used in conjunction with a Varian Techtron Model 63 carbon rod atomizer consisting of a power supply, gas control unit, and a workhead. Nitrogen was used as the protective inert gas a t a flow rate of 3.5 l./min. The power supply and gas control unit are similar to those of the Model 61 (6) but two different graphite sample cells-tube and cup-replace the original rod (4). Figure 1 shows the tube and cup held between graphite support rods. The support rods are clamped in the terminals of the workhead-one of the terminals being fixed, the other being moveable. Efficient use of the heating power available is ensured by the contact resistance between the support rods and the sample cell. The support rods were made of RWO spectrographic grade graphite (Ringsdorff-Werke GmbH, Bonn-Bad Godesberg, W. Germany). Sample cells were either RWO or pyrolytic graphite coated PTlOl (Ultra Carbon Corp., Bay City, Mich.). Solutions were introduced into the sample cell with a microliter pipet incorporating a Chaney adaptor and disposable Teflon tips (Diagnostics Division, Pfizer Inc., New York). The AA-5 spectrophotometer with an air-acetylene flame was used for the comparative study. Air Sampling Equipment. A modified cup with a perforated base as shown in Figure 2 was used to collect air samples. The base was covered with a disk of M F Millipore filter GSWP (Millipore Filter Corp., Bedford, Mass.) having a mean pore size of 0.22 pm. A tttanium punch was used to cut the disk 2.8 mm in diameter. As an alternative to the perforated cup with Millipore filter, a study was made of an XA-3 porous grade graphite cup (Poco Graphite Inc., Decatur, Texas) having an average pore diameter of 1.4 pm. Air samples were filtered through the cups with a spring loaded suction pump with preselectable volumes of 50, 100, and 200 ml. The cup was inserted into a Teflon adaptor and seated on an O-ring in the base. Cups were stored in a graphite sample rack with a plastic cover. Procedure. In order to reduce the blank lead level, the Millipore filters were treated with 1:2 nitric acid and washed with distilled water before being cut into disks. After collecting the sample, 2 pl of 1000 ppm phosphoric acid was deposited on the filter in the cup. The cup was then inserted between the support rods, clamped, and the preselected sequence of drying, ashing, and atomization was carried out. The optimum parameters were as listed in Table I. Aqueous lead standards of 2-pl volume wera deposited into the standard cup or the perforated cup containing the filter followed by the addition of 2 pl of 1000 ppm phosphoric acid. When using the RWO cup, a lower drying voltage was used (0.7 V) qnd the solutions were syringed into the sample cavity while the drying step was operating, thus slowly evaporating the solvent upon contact with the graphite surface. This procedure was necessary to prevent the solutions from soaking into the graphite with consequent reduction in the atomic absorption signal. This precaution was not necessary while working with P T l O l cups as the pyrolytic graphite coating is impermeable to solutions. The analytical signal for lead was measured from the peak height recorded during the atomization step corrected for the blank reading. The samples for the comparative study by flame atomic absorption were collected by filtering the air through the 25-mm di(3) S. H . Omang, Anal. Chirn. Acta, 55,439 (1971). (4) M . D. Amos and J. P. MatouSek, Pittsburgh Conference on Analyti-
cal Chemistry and Applied Spectroscopy, Cleveland, Ohio, March
1972. ( 5 ) R. Woodriff and J. F. Lech, Anal. Chem., 44,1323 (1972). (6)K . G. Brodie and J . P. Matousek. Ana/. Chem., 43, 1557 (1971)
n
9 mm
I
*
pi 2.5 mm D i a . 4
Dia.
+ -3mm
Figure 1. Graphite sample cells
7- - r
?r
r
5 Holes 0.35 rnm Dia. '
Figure 2. Modified graphite cup for air sampling
Table I . Operating Parameters RWO cups Time, sec
Voltagea
Dry
Step
14
Ash
a
Atomize
3
1 0 15 32
PT101 cups
Time, sec 14
Voltagen
a
3
1.2 1 7 35
Actual voltages across the workhead terminals measured digital voitmeter
with
a
ameter M F Millipore GSWP filter placed on a sintered glass frit. The air was filtered a t a rate of 16.2 l./min for 30 minutes using an electrically driven pump. The filter with the particulate matter was digested with 2 ml of concentrated nitric acid and evaporated to near dryness. A further 2 ml of the acid were added and the evaporation was repeated. The resulting solution was transferred to a 10-ml volumetric flask and made up to the volume with distilled water.
RESULTS AND DISCUSSION During selection of a suitable filtering medium for particulate lead, the porous XA-3 graphite cup was studied. In spite of the relatively large pore size, the sampling time for 200-ml volume was between 3 and 5 minutes. The major drawback, however, was the poor absolute sensitivit y of 1.1 x 10-10 gram of lead (for 1% absorption). This
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 9, A U G U S T 1973
1607
1.2 I n
8
v
Time (sec) Figure 3. Oscilloscope traces of lead atomic absorption signals at the 217.0-nm line from untreated particulate ( a ) and particulate treated with phosphoric acid ( b ) Om5
1
a
i
r
I_ 0
30
Time (sec) Figure 4. Recorder traces of air samples at 217.0 nm with a lead lamp (a) and hydrogen continuum lamp ( b )
0
0
5
10
15
20
Pb (u9/m3) Figure 5. Analytical working curves at the 217 0-nm lead line using a 200-ml sample 0 , PT 101 sampling cup, 0, PT 101 standard cup
observation could be attributed to rapid diffusion of lead atoms through the hot porous graphite walls. Since Millipore filters have been successfully used for collection of particulate matter, the combination with the RWO or P T l O l graphite sampling cup appeared to be promising. While selecting the pore size, consideration was given to a report on particle size examination ( 7 ) which indicates that most lead particles are below 2 pm in diameter while many are smaller than 0.2 pm. Conse( 7 ) W C McCrone Amer Lab 3 ( 1 2 ) 8 (1971)
1608
20
40
60
80
100
Pb ( v g / m 3 ) Figure 6.
Analytical working curves at the 283.3-nm lead line
using a 200-ml sample 0 ,RWO sampling cup; 0, RWO standard cup
b
Atomic
krPeak
"0
quently, a filter having a mean pore size of 0.22 pm was used throughout this study. Up to fourteen different lead compounds have been identified in atmospheric particulates ( 7 ) . For this reason, it has been found necessary to add an excess of phosphoric acid to the collected sample before analysis. An oscilloscope trace taken during the atomization step showing the lead absorption of two air samples appears in Figure 3. In the sampIe containing added phosphoric acid, there is a single absorption peak; whereas in the sample without phosphoric acid, there are multiple peaks. By checking a wide variety of samples with a hydrogen continuum lamp, it was found that these peaks were due to atomic absorption only. Figure 4 shows the recorder trace of a typical analysis ( a ) together with the trace for a similar sample recorded using a hydrogen continuum lamp ( b ) . It is clear that the first two peaks are non-atomic. They are caused by decomposition of the filter material. It has been assumed that the lead particulate from the air sample when treated with phosphoric acid is converted mainly to lead phosphate. This assumption is supported by the fact that the atomic absorption peaks for the air sample and for the aqueous lead solution (treated in the same manner) occur a t virtually the same time and, consequently, the same temperature during the atomization step. Therefore, while developing a method for standardization, lead phosphate was prepared on the filter in the sampling cup from aqueous lead with the same excess of phosphoric acid as used for the particulate. In order to avoid the need for a sampling cup with filter for the standardization, aqueous lead solutions were deposited in the standard cup followed by the phosphoric acid addition. The analytical working curves thus obtained were compared with those for the sampling cup with filter (Figures 5 and 6). The maximum difference between the curves amounts to 5.5% for the 217.0-nm line and 10.5% for the 283.3-nm line. The absolute sensitivities achieved at the 217.0- and 283.3-nm lead lines used for the calibration were 1.9 X 10-11 gram and 4.5 x 10-11 gram, respectively, for the RWO cup, and 1.7 X 10-11 gram and 4.0 X 10-11 gram, respectively, for the P T l O l cup. In terms of the concentration of lead using a 200-ml volume air sample, this sensitivity represents for example, 0.1 pg/m3 of lead with the RWO cup a t the 217.0-nm line. The relative standard deviation calculated from three series of 15 measurements of
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 9 , AUGUST 1973
50
VENT TO
I
ATMOSPHERE
EXHAUST
.\
REDUCED CROSS SECTION
Y
BLOWER
Figure 8 . System for producing lead particulate matter in air
Table II. Lead Concentrations in the Air in the Melbourne Metropolitan Area Location
@g/m3of Pb
City, inside car in moving
traffic Time (min)
Figure room
7.
4.0
City, inside car in stationary
Time variation of lead concentration in the enclosed
a solution containing 2 x 10-10 gram of lead (equivalent to 1pg/m3 of lead in a 200-ml air sample) was 4.2%. In order to estimate the accuracy of the method, a static atmosphere containing lead particulates was created in an enclosed room of 98 m3. Automobile exhaust fumes were pumped into the room for a period of 15 minutes. After a 5-minute delay allowed for equilibration, the collection of particulates on the 25-mm diameter filter was commenced. At the same time, and then a t subsequent 5-minute intervals, 200-ml samples were collected with the sampling cup. The variation in the lead content with time thus obtained is shown in Figure 7. These values are based on the calibration using the sampling cup, and the average lead concentration of 7.7 pg/m3 was obtained after integrating the area under the curve (7.2 pg/m3 when calibrated using the standard cup). This result is in good agreement with 6.3 pg/m3 of lead found by flame AA analysis of the particulates collected on the 25-mm filter during the 30-minute interval. The precision of the method was established by taking replicate samples from a dynamic system which produced an atmosphere of constant concentration of lead particulates. This system, shown in Figure 8, consisted of an air blower delivering 240 1. air/min through 4 m of tubing 150 mm in diameter into a box of volume 0.53 m3. Exhaust gases were fed through a “Y” piece into the tubing and mixed with the air. The “Y” piece was connected to the vehicle exhaust with one branch (16-mm diameter) being vented directly to atmosphere; the other (diameter reduced to 2.5 m m ) was connected to the system. The vehicle was run slightly above idling speed and a period of 20 minutes for equilibration was allowed before sampling commenced. Samples were taken inside the box via the sampling port. Three series of 15 measurements yielded relative standard deviations of 6.7, 7.2, and 8.3% at concentrations of 15.0, 11.3, and 9.5 pg Pb/m3, respectively. The difficulty of producing a constant concentration of lead particulates with the simple system used means that these figures reflect not only the reproducibility of the sampling proce-
traffic City, intersection City, intersection partially enclosed by higher buildings City, underground car-park City, road between high buildings S u b u r b , intersection S u b u r b , road
Suburb, residential area
6.5 0.7 to 3.4
0.9 to 4.6 9.5
5.1-6.2 0.7-4.4 0.5-2.7 0.2
dure, but also possible time variation of the particulate concentration. This may explain why better relative standard deviations are obtained for aqueous solutions than for the air samples taken from this system. The accuracy test as carried out with the static atmosphere in the enclosed room was repeated with the dynamic system described. A relative accuracy of 17.5% was obtained for the sampling cup procedure when compared with the value of lead found by flame AA analysis of the particulates collected on the 25-mm diameter filter. The results of measurements taken during August 1972 in the Melbourne metropolitan area are summarized in Table 11. The range of the results is similar to those obtained by a number of methods at different locations (3, 5, 8 ) . The proposed technique enables a virtually instantaneous monitoring of airborne lead particulate concentration since, for example, it takes only about thirty seconds to collect particulate matter from a 200-ml air sample. A distinct advantage of this method is that the possibility of contamination is reduced since handling of the collected sample is minimized and only lead present inside the cup can contribute to the analytical signal.
ACKNOWLEDGMENT We thank I. R. Bartlett for the construction of the sampling equipment. Received for review October 25, 1972. Accepted February 5, 1973. (8) G. B. Jackson and H. N. Myrick,Amer. Lab., August, 19 (1970)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9 , AUGUST 1973
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